This invention relates to an air generation unit (AGU) suitable for an aircraft, and more particularly, the invention relates to a pack and a half air generation unit configuration utilizing two air cycle machines (ACM) with each preferably having two turbines.
AGUs typically include at least one ACM and at least one heat exchanger that receives air from a pressurized air source, such as bleed air from an engine, to provide cooled air to the aircraft cabin and cockpit. The AGUs may be packaged within the wings and/or tail section of the aircraft. Accordingly, it is desirable to provide an AGU having a very small package to limit the amount of aircraft structure that must be removed to accommodate the AGU. Furthermore, the AGU must provide sufficient cooling for the size of the aircraft.
A pack and a half AGU has been developed for use in a Dash 8-400 in which two ACMs share a common heat exchanger. The pack and a half configuration provides increased cooling and smaller packaging than two separate AGUs each having their own heat exchanger. The Dash 8-400 utilizes a three wheel air cycle machine having a fan, a compressor, and a single turbine.
The Dash 8-400 AGU includes a heat exchanger having primary and secondary heat exchangers. Bleed air is taken from an intermediate or high pressure stage of a turbine engine. The bleed air is pre-cooled within the primary heat exchanger with the heat being rejected to ram air and then communicated to the compressor of the ACM. After compression, the air is communicated through a secondary heat exchanger to a condenser. Condensed water vapor is extracted by a water collector, and the dehumidifier air is sent to turbine where the air is expanded to generate cold air. The cold air is sent to an environmental control system (ECS) that further conditions and distributes the air to the aircraft.
A Boeing 777 utilizes AGUs with a single ACM and single heat exchanger. The Boeing 777 ACM is a four wheel configuration that includes a fan, a compressor, and first and second turbines. The operation of the AGU is similar to the Dash 8-400 AGU, but the second turbine receives the cold air from the first turbine and further expands the cold air to produce subfreezing air. The second turbine produces air sufficient to cool larger aircrafts.
Very large commercial aircrafts are being developed capable of carrying up to 1,000 passengers or more. These large aircraft require AGUs capable of producing very cold temperatures. However, the design constraints for the aircraft require very small packaging with very high reliability. The Dash 8-400 AGU configuration produces conditioned air that is not sufficiently cold for such a large aircraft. The Boeing 777 AGU configuration provides sufficiently cold air, however, up to four or more AGUs would be required for such a large aircraft, which would necessitate removing significant aircraft structure and would significantly increase weight. Therefore, what is needed is an improved AGU having a small package, increased reliability, and sufficiently cold air for the needs of a large aircraft.